Looking to modify a transistor input stage (LTP), and thought that adding a current source using a CRD would be an interesting thing to experiment with.

Seems simple...remove the common emitter resistor, replace with a two-lead CRD rated for the desired current.

Besides the PITA to find these devices, I can see no real reason why they are not used more often. A current source for a LTP is so much superior to a resistor for biasing, and the easiest way to go about it is using a CRD. So I can't help but wonder why I do not see these devices used as I have described....?

Anyone try designing using CRD's? Problems?

Lastly, has anyone got a source for the Vishay CRD's? Excellent specs and very low output capacitance (6pf or so), but no one seems to carry them....

Originally posted by EchoWars Looking to modify a transistor input stage (LTP), and thought that adding a current source using a CRD would be an interesting thing to experiment with.

Seems simple...remove the common emitter resistor, replace with a two-lead CRD rated for the desired current.

Besides the PITA to find these devices, I can see no real reason why they are not used more often. A current source for a LTP is so much superior to a resistor for biasing, and the easiest way to go about it is using a CRD. So I can't help but wonder why I do not see these devices used as I have described....?

Anyone try designing using CRD's? Problems?

Lastly, has anyone got a source for the Vishay CRD's? Excellent specs and very low output capacitance (6pf or so), but no one seems to carry them....

Are you talking about constant current diodes?

If you are maybe they are not too much used because they are expensive, not easy to find and not as precise as they should be.

A FET + adjusting resistor, preset and hand picked for exact current, might be a better alternative to what you proppose. For the same objective. Maybe.

Using JFETs + 1 resistor is much more flexible.
Those diodes are in fact JFETs, but you can not change the current,
as only the output pins are available.
-----------------------------------------------------------------www.elfa.se has some current doides for different mA.
But in order to work they need >= 10 Volt across them.
That will make them unsuitable for some applications.
The diodes, E101 ...., also have wide tolerance +-20% from nominal value of milliampere.
example:
E202 mA= 1,68-2,32; Vf= 10-100Volt; Rdiff= 0,25MOhm
E103 mA= 8,00-12,0; Vf= 10-100Volt; Rdiff= 0,17MOhm

Just like zener diodes, CRD's can be extremely convenient tools, and if you know how to deal with them, they can be useful for low-level as well as high-level circuits. However, they have certain quirks that should be taken into account. Careful studying of the catalogs (including the curves) should give you some idea of how these devices should be used (and also how they should _not_ be used).

Seems simple...remove the common emitter resistor, replace with a two-lead CRD rated for the desired current.

It is simple. Why not use the tool designed for the job? You get excellent temperature stability, low capacitance - typically just a couple of pF, operation from 2 or 3V to 50V and fairly high Z (>200kohms).

Originally posted by EchoWars Well, finding a manufacturer is a hell of a lot easier than finding someone to sell you a few. I found a lot of devices online at manufacturer sites, but pretty damn few sellers.

The Vishay parts (CR160 et al) look nice, but I have yet to find a seller. Pretty tight tolerances on these.

If I decided to 'roll my own', might it be better to skip the JFET idea, and simply use a bipolar? I am still attracted by the two-pin CR160 series, but I hate to sacrifice noise parameters...

It is nice to have 2 pin device.
And the JFET or CC-diode has one other obvious advantage.
It does not have to use a separate power supply.
It is only supplied by the current, that runs through it.

A bipolar (or a MOS) needs a reference voltage.
This can be made out of a red LED, another bipolar used
as current limiter, or some other reference.

This reference needs a supply.
This is usually taken via resistor from Power Supply.
And to avoid noise/hum from the supply
you can need to use filter caps Elyts and/or Film caps.
The need is depending on how clean the supply is.

But as the LED and bipolar smallsignal has low noise
and good temp-matching,
this can give very good, clean and very constant current.
-----------------------------------------------------------

So diode/JFET method is simple and does not need a ref with PSU.
It makes it isolated from Power Supply Rail disturbance.

LED + bipolar, or a bipolar with another bipolar as current limiter,
gives very clean low noise.
But they need contact with PSU-rails, and can require
heavy or less filtering, to decouple from PSU.
Apart from that you will never find any problems
to get a hold of LEDs or small-signal Bipolar Transistors

/halo - uses bipolar Constant Current Sources
(because he has not many JETs around, and not CCS-diodes)